This invention relates to an apparatus and process for treating particulate materials or powders within a rotating retort, and more particularly to such an apparatus and process in which gas is supplied to the retort to fluidize the particulate material within the rotating retort.
U.S. Pat. No. 5,407,498 dated Apr. 18, 1995 is directed to an apparatus having a retort mounted for rotation about a horizontal axis and containing a particulate material therein which is fluidized while the retort is rotated. The retort is mounted in a cantilevered relation from an axle secured to one end of the generally cylindrical retort. Gas conduits extend through the end axle and gas may enter the retort and be exhausted from the retort through the gas conduit. Filters on the ends of the conduits prevent the flow of solid particles or particulate material into or out of the retort. Thus, particulate material cannot be loaded into the retort or unloaded from the retort while the retort is being rotated. Further, even when the retort is not being rotated, an end cap is required to be removed in order to provide access to a port for loading or unloading the particulate material. Also, the filters shown on the inner ends of the gas conduits within the retort are easily clogged with particulate material embedded within the filters.
It is desirable to have a retort which may be easily loaded with particulate material and unloaded in a minimum of time and without any loss of the particulate material by leakage or the like. It is particularly desirable to have such a loading and unloading means which may be utilized during operation of the retort while the retort is rotated.
As set forth in U.S. Pat. No. 5,407,498 metallurgical operations rely on the movement of certain elements within the solid matrix of the metal to be treated. Metallurgical operations rely on chemical reaction between elements which may be physically brought together or may be induced to come together by diffusion. An element is any chemical element or substance listed in the periodic table. Elements move within the solid metal by a process of diffusion. Diffusion is encouraged when an element is attracted to another element with which it is more reactive within the same contiguous metal structure. Diffusion also occurs when metals tend to form a more homogenous solid solution. Diffusion of an element from one metal to another or between a gaseous atmosphere and a metal also takes place when the materials are in intimate contact. The employment of precise pressures is often desirable to assist in this transfer.
As an example, fine metal particles or powders of nickel and aluminum can be intermixed and brought to a temperature well below the melting point of either metal and they will react to form a nickel aluminide intermetallic compound. The combining of nickel and aluminum powders produces substantial amounts of heat. This heat can raise the temperature of operation, which further speeds the combination and eventually an uncontrolled or runaway operation can happen. Temperatures can be produced which are sufficient to melt the powders so that they agglomerate together in an undesirable mass.
Water atomized iron particles or powders which contain excess carbon and oxygen can be reduced by a combination of diffusion within the powder and chemical reactions at the surface. The oxygen is primarily an oxide on the surface of the powder but the carbon is diffused throughout. By bringing the powders into intimate contact in the pressure of hydrogen, a reaction is generated at the surface whereby carbon and oxygen combine to form a carbon oxygen gaseous compound and hydrogen and oxygen combine to form gaseous water. Carbon within the powder migrates by diffusion to the surface and reacts with the oxygen. Heat must be added to initiate the reactions and good thermal transfer helps maintain constant temperatures which are important for a controlled reaction. In some cases, the reaction changes from endothermic to exothermic as the carbon is dissipated and hydrogen begins to combine directly with the remaining oxygen.
Ideally, the elements in the fluidizing process to undergo a reaction will be brought into intimate contact with each other and will be held in contact for sufficient time for the chemical reaction to take place. Further, it is important that if heat is to be added during the reaction, it must be added with great uniformity so that the reaction takes place at the desired temperature. In the case of those reactions which generate heat it is even more important to have good thermal transfer so the heat can be removed from the operation to avoid an undesired rise in temperature.
The process or method as shown in U.S. Pat. No. 5,407,498 is not concerned with maintaining the exothermic or endothermic reactions of the particulate material at a precise uniform temperature during fluidizing within a rotating retort by (1) precisely controlling the addition of heat to the retort or the release of heat from the retort to match the heat loss or gain to or from the retort resulting from chemical reactions within the retort, and (2) precisely adding or injecting another material into the rotatable retort during fluidizing of the initial particulate material in the retort so that heat generated or lost matches the heat induced to or exiting from the retort as a result of said injection of particulate material.
The apparatus of the present invention is directed to apparatus for treating two types of workpieces. Workpieces may be particulate material such as metal powders. Workpieces may also be solid parts, which are placed amongst particulate materials.
The term xe2x80x9cworkpiecexe2x80x9d as used in this specification and claims is interpreted as a powder or a solid part which is the subject of the treatment. The term xe2x80x9cpowderxe2x80x9d or xe2x80x9cparticulate materialxe2x80x9d as used in the specification and claims is interpreted as small particles of material having a size less than 1000 microns. The term xe2x80x9csolid partsxe2x80x9d as used in the specification and claims refers to materials of a specific fixed shape having at least one dimension greater than around 1000 microns.
Workpieces can be either solid parts or powders. When the workpiece is a solid part, the powder which is selected for the workpiece to be placed amongst is generally inert to the process and its functions comprise heat transfer, scrubbing and intermixing. When the workpiece is powder, the powder still fulfills the functions of heat transfer, mixing and scrubbing but is also the object of treatment.
Treatments are carried out in a retort, mounted for rotation about a generally horizontal axis. The retort may be heated or cooled by gases transported to the interior of the retort through a fluid passage and an axle on which the retort is mounted for rotation about a longitudinal axis. The retort is preferably supported on a tilt frame to permit the retort to tilt in a vertical plane about a horizontal axis so that particulate material in the retort may flow by gravity into and out of a desired end of the retort upon tilting of the retort to a predetermined tilt angle.
The enclosed retort is sealed from atmosphere and mounted on a pair of aligned end axles for rotation. Flow lines or flow passages into and out of the retort are provided through the axles. Gases and particulate material may be injected into the retort and exhausted form the retort as desired while the retort is rotating. Solid parts may be placed within the retort amongst the powders or may be fixtured to rotate with the retort. Flow conduits including filters are provided in one end axle to inject gases into the retort and exhaust gases form the retort. The flow of the gases through the conduits may be reversed and this is effective for minimizing any clogging of the filters. The particulate material or powder is injected through a conduit in the other aligned end axle. A vacuum is often used for the exhaust of particulate material from the retort. Valve means for the conduits effectively control the flow of gas and particulate material into and out of the retort through the conduits in the axially aligned end axles. Each conduit includes a fixed conduit portion connected to a swivel for the rotating axle and a rotatable conduit portion extending through the axle and communicating with the interior of the retort.
A detachable container for particulate material may be mounted on an end axle to supply particulate material to the retort. The detachable container which is not normally mounted for rotation with the axle may be removed after injection of particulate material within the retort and another container connected to the axle to receive particulate material from the retort if desired. The particulate material within the container may be fluidized for ease of movement within the interior of the container. The particulate material injected into the retort from the separate container may be cooled to a predetermined low temperature, prior to injection, if desired, thereby acting to quench hot solid parts placed within the retort.
The retort is preferably mounted on a pair of axially aligned end axles, one on each end of the generally cylindrical retort. A gas conduit is positioned in an end axle and a solid particulate conduit is positioned within the other end axle. Valve means for the conduits are mounted for rotation with the axles to control the flow of gas and solid particles within and out of the retort.
The present invention provides means for loading and unloading particulate material in a minimum of time while the retort is rotating and with minimal loss of the particulate material. The particulate material may be easily injected during operation of the retort. A sample of the particulate material within the retort may be easily removed during operation of the retort for suitable testing or the like.
The process of the present invention is directed to a process for mechanically fluidizing small metallic particles within a retort mounted for rotation about a horizontal axis, and particularly to such an process which includes the isothermal control of an exothermic or endothermic reaction of the small metal particles with another material injected into the rotatable retort. An endothermic reaction is a process or change that takes place with the absorption of heat whereas an exothermic reaction is a process or change that takes place with the creation or evolution of heat. It is desired that such an endothermic or exothermic reaction be controlled under a uniform or constant temperature, (i.e. an isothermal control). The temperature of the reaction can be controlled by adding or removing heat as necessary during rotation of the retort. A precise isothermal control may be maintained during the exothermic or endothermic reaction.
The material injected into the retort during rotation may comprise a particulate material and the rate of injection for particulate material also controls the rate of reaction between the injected particulate material and the initial particulate material already in the retort.
A mechanically fluidized retort provides the heat transfer, intimacy and residency with little gas flow because the fluidization does not require the passage of gas through the material. Fluidization is mechanical so the gas may stay in residence within the fluidized mass long enough for the desired reactions to take place. The fluidization action results in near constant movement of particles relative to each other so they do not stick together even at relatively high temperatures. Nevertheless, the heat transfer rate in a mechanically fluidized device is sufficient to control the temperature of the reaction by adding or removing heat as necessary. The injection of material into the retort, particularly particulate material, during rotation of the retort and fluidizing of particulate material within the retort, is at a predetermined controlled rate to control the rate of reaction between the particulate material within the retort and the material being added to the retort. An important part of the mechanical fluidizing system is the relatively long residence time of gas and powder. Although gas and powder are completely intermixed, the gas stays in the retort for a relatively long time compared to gas fluidizing systems, assuring complete reactions without resorting to re-circulation systems.
It has also been found possible to further control the rate of reaction by maintaining a totally inert atmosphere until a predetermined temperature is reached after which a reactive gas or powder is injected to create or abet the desired reaction within the retort. By combining the very precise control of injection of reactive material with the excellent thermal conductivity created within the fluidized mass, it is possible to control reactions such as the reduction of copper which has proved extremely difficult in previous equipment which does not combine the controlled introduction of reactive material such as hydrogen with the temperature uniformity capable in a fluidized mass.
Thus, the present invention provides an unexpected capability to maintain a precise isothermal condition for exothermic and endothermic reactions. The process and apparatus may be utilized for the treatment of various small metallic particles. For example, aluminum particles may be utilized to coat small nickel particles or powders. Another example is in the reduction of the oxygen content and carbon content of small iron particles.
Other features and advantages of the invention will be apparent from the following specification and drawings.